As the death toll from the Ebola outbreak exceeds 1,000, pharmaceutical companies and government agencies are rushing to create the first-ever drug for the illness.

But it is not the only tropical, infectious disease that lacks a drug. Due to some of the same economic and logistical reasons that have discouraged industry investment in Ebola, diseases such as dengue and West Nile virus also have no cures or vaccines - even though they afflict significantly more people. Some 50 to 100 million people are infected with dengue every year, and 5,600 West Nile virus cases were reported in the United States in 2012, the highest number in almost a decade.

Industry has not entirely ignored tropical diseases. There are dengue vaccines in the works by pharmaceutical giants Sanofi and PaxVax in Redwood City. Potential vaccines for West Nile virus have been tested in humans. There are preventive drugs for malaria, and a yellow fever vaccine.

But drugs for tropical diseases have been slow-coming in general, in part because they are risky and hard to study. Financial challenges include funding research and finding a market. Ebola's sporadic outbreaks and infectious nature have made companies wary of researching it, although some are now testing experimental therapies in response to the outbreak.

"Existing solutions are often ineffective, unaffordable and inappropriate for the needs of the resource-poor populations affected" by more than a dozen neglected tropical diseases, a 2013 World Health Organization report said. "There is an urgent need for new or improved vaccines, diagnostics and treatments for these diseases."

The obstacles to drug development vary among diseases. For dengue, the need is clear: The World Health Organization estimates half the world's population is at risk.

Historically present in tropical and subtropical climates, dengue is now making its way into the United States, probably because the mosquito that carries it is adjusting its flight patterns because of climate change. Last year, the insect appeared in California for the first time.

In 2012, PaxVax lost its first bid for dengue vaccine funding from the National Institutes of Health, which was facing budget cuts because of the recession. The company was again denied funding in 2013. This year, CEO Ken Kelley is crossing his fingers.

"It's starting to get better, but it's still very, very hard," he said of government funding. "That's why the program was going so slowly."

For West Nile virus, on the other hand, a vaccine is easy to find - for horses. No approved vaccines for humans exist, although some are in clinical trials.

"The main reason for that is that pharmaceutical companies claim that the market is limited for a human vaccine, and the seasonal and unpredictable nature of the infection makes the setup of clinical trials difficult," French researchers wrote in 2013 in the scientific journal Viruses.

First documented in the United States in 1999, West Nile virus is usually transmitted through infected mosquitoes, which pick up the virus by feeding on infected birds. The disease is tough to diagnose because 80 percent of infected people show no symptoms, and it can lead to neurological disease and death.

Last week, the virus was found in birds and mosquitoes in Contra Costa County. State health officials also said this month that two people have died of the virus in Northern California, the state's first such deaths this year. Without a vaccine, the only ways to reduce infection in people include using mosquito nets, wearing repellent and avoiding the outdoors at peak biting times.

Another tropical disease that lacks a cure or vaccine is chikungunya, a mosquito-transmitted disease that appeared in Florida for the first time this summer and has infected an estimated 135,000 people in the Caribbean this year. The Marbug virus, which has symptoms similar to Ebola and is also rare, has no treatment or vaccine.

Some tropical diseases have success stories. Onchocerciasis, the parasitic disease known as "river blindness," has no vaccine, but it has a medicine that public health officials have used to cure millions of Africans.

In general, companies that want to serve at-risk populations must balance those desires with their checkbooks. PaxVax, for example, believes its dengue vaccine could be more convenient to take than the one Sanofi has spent $1.5 billion and 20 years on.

But PaxVax's vaccine, which has only been tested in animals, is far behind other vaccines for HIV and anthrax. So the privately held company needs to invest in its most promising products first, Kelley said.

"It all depends on financing," he said. "With a small, struggling biotech company, we're dependent on raising funds to advance different development programs."